1. Field of the invention
The present invention relates to a control device for servo motors used to drive industrial robots, etc.
2. Description of the Prior Art
In order to move a top portion of an industrial robot, such as an arm thereof, along a predetermined passage at a desired speed by a servo motor, it has been usual to control the servo motor on the basis of objective values of a position or angle to which the arm is to be settled, and a moving speed of the arm, which is obtained by calculations. In a conventional control device for driving such an industrial robot, a positional deviation .DELTA.P is obtained by integrating a difference .DELTA..omega.=.omega.*-.omega..sub.m between a measured value .omega..sub.m of rotational speed of the servo motor for driving a mechanical system composed of the arm, etc., and an objective rotational speed .omega.* thereof, and a reference value .omega.p* of the rotational speed of the servo motor at which the positional deviation .DELTA.P becomes zero, is obtained in a position control device by multiplying a signal indicative of the positional deviation .DELTA.P with a predetermined gain K.sub.i. Then, a reference torque value T* of the servo motor at which a difference between the reference speed value .omega.p* and the measured speed value .omega..sub.m of the servo motor becomes zero, is obtained by a speed control device. An armature current i of the servo motor is regulated by a power converter on the basis of the reference torque T* to drive the mechanical system of the robot. Gains of respective control elements are regulated such that dynamic characteristics of the mechanical system become optimum.
In the case of a robot having multiple articulates, the moment of inertia of the arm around the root portion thereof is largely due to the cantilever type of support structure, and the natural frequency of vibration becomes smaller, to facilitate vibration of the mechanical system comprising the arm, etc. In order to restrict such vibration, it has been usual to make the gain K.sub.i of the position control device smaller to thereby reduce the control response or to make time dependency of variation of the objective input .omega.* smaller.
In the former solution, not only does the operation time of the mechanical system becomes longer, but also the positional deviation during operation increases, resulting in the accuracy of continuous interpolation for moving passage being lowered.